I've found some cheap return flights over to the Dominican Republic later this month on Thomson Airways. Departing Gatwick on 14th September and returning 5th October. Flights are direct to Punta Cana.
Other dates i've found:
14th Sept - 4th October
14th Sept - 6th October
14th Sept - 7th October
15th Sept - 6th October (increases to £225)
16th Sept - 7th October (also £225)